Recently, the development of optical interconnecting techniques using light for short-range communication such as between backplanes, has come under close attention accompanying capacity enlargements and improvements in transfer speeds in supercomputers and servers. A vertical cavity surface emitting laser (VCSEL) diode that allows direct modulation while being compact and exhibiting reduced power consumption is used, for example, as a light emitting element in an optical modulator for optical interconnection.
FIG. 1 illustrates a conventional circuit for high-speed direct modulation of a VCSEL.
Normally in an analog circuit, signal transferring is conducted through a differential amplifier circuit as a noise countermeasure. As a result, in the light emitting element drive circuit in FIG. 1, differential drive signals are applied to terminals in and inx and a differential output appears at terminals out and outx. A VCSEL 12 is coupled to the terminal out, and a dummy load 11 having similar impedance characteristics as the VCSEL 12 is coupled to the terminal outx. Furthermore, current sources 10-1 and 10-2 for applying a bias current to the VCSEL 12 and the dummy load 11, are respectively coupled to the terminals out and outx.
The conditions expected in a light emitting element drive circuit for realizing high-speed modulation are, firstly, the ability to perform impedance matching for controlling the reflection of signals and, secondly, the ability to generate a large bias current for driving the VCSEL.
While the conventional example illustrated in FIG. 1 is a configuration that meets these two conditions, there is a problem that the bandwidth deteriorates due to parasitic capacitance of the transistor since the size of a transistor to configure the current sources 10 is preferably large in order to generate the large bias current that is the second condition.
FIGS. 2A and 2B are diagrams for explaining a conventional current source.
FIG. 2A illustrates a current source of the conventional structure. This structure is a so-called current mirror configuration, and as illustrated by the dotted line in FIG. 2B, the structure uses a saturation region (a region in which the current flowing to the drain ideally does not change with respect the drain voltage, but actually has a slope as illustrated in FIG. 2B) of a transistor Tr1. As a result, a stable current supply can be achieved without relying on the power source voltage.
As illustrated in FIG. 2A, the value of a current I1 is derived by multiplying the size ratio between transistors Tr0 and Tr1 by a current I0. Moreover, a drain voltage Vds1 of the transistor Tr1 expressed by the characteristics of the transistor in FIG. 2B is desirably larger than a voltage Vds_sat having a saturated current value and expressed by the transistor characteristics in FIG. 2B, and is desirably smaller than the difference between the voltage Vout for driving the VCSEL 12 and a power source voltage VDD.
However, when an adequate current I1r for driving the VCSEL 12 is sought, the bandwidth deteriorates due to the effect of the parasitic capacitance since the transistor size of the transistor Tr1 is desirably increased in order to increase the saturation current.
Japanese Patent Laid-open No. 2010-56918 describes an amplifying circuit that is used as a driver for an optical modulator, is normally operated in an amplifier output stage, and is able to supply an output level drive signal corresponding to a modulator.